Indirectly heated cathode



April 1-8, 1933.- H. BEDFORD 1,904,261

INDIRECTLY HEATED CATHODE FiledMax-ch 16, 1931 F/G./ FIG? F163 A T TORNEY Patented Apr. 18, 1933 I LIN-tree STATES PAreN'r LESLIE HER-BERT BEDFORD, OF LONDON, ENGLAND,A.SSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF'NEW YORK, N. Y., A CORPORATION OF NEW YORK INDIRECTLY .HEATED CATHODE Application ru s March 16, 1931. Serial No. 522,341, and in Great Britain August 1, 1930.

This invention relates to indirectly heated cathodes for electron discharge devices.

Indirectly heated cathodes" usually comprise a tungsten or other filament which may bestraight or-of-hairpin formation, located within a tube'of insulating material, such as porcelainor silica, which isin turn surroundediby a metallic cathode sleeve.

Although cathodes of this type possess many advantages over the usual .form of directly heated cathodes it'has been foundthat, unless there is goodthermal contact between thecomponent parts, difiiculties are likely to arise and the chief object of the. present invention is'to provide for the obtaining of improved thermal contact between such parts.

According to the invention the residual in connection with the accompanying draw- I Fig. 1 shows thelength of heater wire after crimping and bending with parallel terminating uncrimped portions.

Fig. 2 is a sectional view of a twin-bore insulating tube in which the heater is enclosed.

Fig. 3 illustrates in section the metallic cathode tube.

Fig. 4 illustrates in cross-section the complete assembly of the cathode in accordance with this invention. j

Fig. 5 is a cross-sectional view of Fig. 4 on the line 5, 5. r

In accordance with one feature of the invention, therefore, the filamentary heater 10 is formed with a series of crimps 11, or the like, having an initial amplitude which is greater thanthe'diameter of the bores of the insulating tube 12 shown in Fig. 2. The filamentary heater 10 is preferably composed of a resilient metal such as tungsten, tantalum,-

molybdenum, or I an alloy of tungsten and molybdenum, and the crimps or other deformations are such that the filament can be inserted in the insulating tube 12 of the cathode while under tension and when the tension is released will bind on the inner surface of the insulating tube 12, the pitch'of the crimping 7 being sufficiently fine-to avoid the possibility of hot spots between bearing points. 7

The heater element may be substantially straight or, if desired, it may take the form of a hairpin each length of which is formed with the crimps 11 or other deformations and the two lengths being accommodated in atwii1-bore insulating tube 12. In some instances the improved thermal contact be tween the filament and the insulating tube may be obtained by the use of aspiral or ble in a crimped filament. However, in the cases of the small cathode, the crimped method has, up to the present, been found preferable in practice.

Where the insul ting tube is composed of silica or a like material which may, for instance,'be five to ten mils internal diameter, precaution must be taken in order to enable assembly of the filament in the tube to be effected without fracturing the tube. A convenient method of accomplishing this is to place the deformed filament under tension which is sufiicient to reduce the amplitude of the crimps or the diameter of the heater to a value less than that of the internal diameter of its corresponding bore in the tube 12 and then place one within 'the'other. For .this

purpose, straight or uncrimped lengths 13. and 1% of the filament may be left at each end thereof and these may be inserted through the silica tube leavingthecrimped or deformed portion clear. The free end of the filament may then be gripped by a suitable form of tensioning meansand when a predetermined tension 'hasbeen applied thereto the silica tube may be slipped over the crimped or otherwise deformed filament so that when the tension is released the filament will bind on the inner walls of the tube 12.

In accordance with another feature of the invention, an improved thermal contact between the cathode and the insulating tube is obtained by suitably deforming or shaping the cathode tube. In the case of a cathode 15 of circular or annular cross-section, the cathode tube may be lirst pressed so that it is caused to assume a slightly elliptical crosssection. The cathode is then repressed between a pair of suitable semi-circular jaws until it returns to its original shape and the insulator 12 is then placed in position therein. On releasing the jaws the cathode tube 15 springs back to its slightly elliptical form and grips the insulator tube 12 around a large portion of its circumference thus establishing an adequate thermal contact.

The advantages obtainable by imp-roving the thermal contact between the component parts of an indirectly heated cathode are many among which may be mentioned, the filament and consequently the insulator temperature is lower for a given wattage, hot spots on the filament and er the insulator are avoided with a consequent diminution in the chances of the burning out of the filament, and the filament resistance is maintained constant. It will also be observed that the construction described also provides an ellicient method of absorbing dillerences of expansion between the elements and is consequently superior to those in which the elements or certain of them are cemented.

What is claimed is:

1. An indirectly heated cathode for an electron discharge device comprising a twin bore insulating tube, a hairpin heater element within said tube, and a metallic electron emitting sleeve surrounding said tube, each of the legs of said hairpin heater element having deformed portions frictionally engaging a wall of a bore of said twin bore tube.

2. An indirectly heated cathode for an electron discharge device comprising a twin bore insulating tube, a hairpin heater element within said tube, and a metallic electron emitting sleeve surrounding said tube, each of the legs of said hairpin heater element having oppositely directed crimp portions frictionally engaging a wall of a bore of said twin bore tube.

3. An indirectly heated cathode for an electron discharge device comprising a twin bore insulating tube, a hairpin heater element having crimp portions exerting expansional force on the inner walls of said twin bore tube, and a metallic electron emitting sleeve exerting compressional force on the outer wall of said insulating tube. V

l. An indirectly heated cathode, according to claim 3, in which the forces exerted by said heating element and metallic sleeve are applied along one diameter of said tube.

5. An indirectly heated cathode for an electron discharge device comprising a twin bore insulating tube, a hairpin heater element within said tube, and a metallic electron emitting sleeve surrounding said tube said sleeve exerting inward pressure on diametrically opposed outer portions of said insulating tube and laterally opposed outer portions of said tube being spaced from said metallic sleeve.

6. A method of assembling a heater type cathode comprising a twin bore insulator, a

hairpin heater element and a metallic cathode sleeve, which comprises crimping the legs of said hairpin heater so that the initial amplitude of the crimps is greater than the diameter or" each bore in said insulator, applying said twin bore insulator to straight portions of said hairpin heater element, tensioning said hairpin heater element, sliding said insulator along the crimped legs of said heater, and relieving the tension of said heater element.

7 The method of assembling a heater type cathode comprising a cylindrical insulator and a tubular metallic cathode, sleeve which comprises deforming said sleeve to an elliptical shape, pressing the sides of said sleeve to return said sleeve to a cylindrical shape, inserting the insulator within said sleeve, and removing the pressure on the sides of said sleeve. 7

8. The'method of assembling a heater type cathode comprising a twin bore insulating tube, a hairpin heater element, and a cathode sleeve, which comprises initially deforming said heater element and cathode sleeve, applying said heater element and cathode sleeve to said insulating tube, and releasing said heater element and sleeve to frictionally connect said heater element and sleeve to the walls of said insulating tube. V v

In witness whereof, I hereunto subscribe my name, this 11th day of February, 1981.

LESLIE HERBERT BEDFORD. 

